GPS Track This sells small, lightweight GPS trackers for motorcycles and bicycles.
The “GPS Track This” device combines cell phone technology with a GPS to send SMS text alerts when the bike has been moved. For bicycles, the GPS tracker is hidden inside of a bicycle tail light that you affix to your seat post. You can also track the location of the GPS unit on the web to help you recover your bike.
The bike unit costs UK£159, or about US$250.